Spent fuel treatment

Proper nuclear waste management is a key issue for the nuclear industry, including research reactors. The citizens of most countries regard it as one of the major concerns relating to the nuclear industry.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 Time Before Start of Operations (years)

Cumulative Percentage of Capital Expenditure

FIG. 11. Sample capital expenditure curve.

An important aspect of nuclear waste is that it has the potential to impact the environment for tens of thousands of years. It is clear then that the choices made today cannot be based solely on short term economic and technical information. A research reactor operator has the same responsibility to define a spent nuclear fuel management plan that satisfies safety and public acceptance requirements as an electrical utility that operates nuclear power generating stations. However, due to the small quantities of fuel used in research reactors, the economic aspects related to nuclear fuel waste management are generally very unfavourable. The economics can be improved significantly if the research reactor operators can make use of an existing spent fuel and radioactive waste management programme, for example, as part of a national policy.

Research reactor fuels have important differences from power reactor fuels. The dimensions, uranium enrichment levels and corrosion resistance of the cladding and fuel material are quite different. The higher residual enrichment and the more corrosion prone aluminium cladding have to be specifically addressed when considering either long term storage or repository disposal. This includes the options for mechanical and chemical treatment of the fuel, and/or encapsulation of the fuel to better adapt it to disposal in geological repositories.

For spent fuel treatment, two main approaches can be considered, reprocessing and conditioning for disposal.

Both processes produce long lived radioactive wastes that eventually need to be disposed of in geological repositories, though the type and the final volume of waste for disposal differs between the two approaches.

Spent fuel reprocessing includes the recovery of usable fissile material from irradiated fuel assemblies, in addition to separation and treatment of the fission product and actinide wastes. For a detailed review of fuel reprocessing, its techniques and applications to different types of fuels, see IAEA-TECDOC-1467 [16].

All of the operating commercial reprocessing plants around the world are based on the plutonium and uranium recovery by extraction method (PUREX) solvent extraction process. This process is capable of recovering more than 99.7% of the uranium and plutonium and of processing high burnup spent fuel. However, the different research reactor fuel types have differing degrees of acceptability to the commercial reprocessing facilities. For example, the widely adopted uranium silicide fuels are more difficult, and therefore potentially more costly, to reprocess than other common fuel compositions.

Other spent fuel reprocessing technologies are under development, but not yet in commercial operation.

Reprocessing of research reactor fuel results in the separation of the non-volatile radionuclides as high level liquid wastes, high level solid wastes (HLW) which are normally returned to the country from which the spent fuel originated, and low level solid and liquid wastes which may be retained by the reprocessor in exchange for a small additional amount of HLW supplied to the owner of the fuel. Section 5.2.2 provides more information on spent fuel reprocessing.

Spent fuel conditioning refers to the treatment of the fuel to achieve physical and chemical characteristics that match the requirements of a repository, and to reduce the fissile content of the uranium as appropriate to satisfy non-proliferation requirements. Several conditioning technologies are under development. Some involve mechanical compaction or comminution of the fuel, followed by mixing with depleted uranium or a neutron poison; others include encapsulation of the fuel in vitrified HLW; or dissolution without reprocessing, followed by dilution with depleted uranium and vitrification. The primary waste stream from conditioning is LILW, rather than the HLW that results from reprocessing, because the thermal output of the conditioned spent fuel is below 2 kW/m³.

5.2.1. Consideration of the national nuclear policy

Any national nuclear policy is likely to play a key role in the decision making process undertaken by a research reactor operator. Obviously, the influence of the national nuclear policy will have a large effect on the technical choices and the economics of spent fuel and radioactive waste management.

If there are other national nuclear facilities (e.g. other research reactors, commercial power generators, etc.) then there is scope for co-operation between the different entities which should result in a reduction in the cost to any one of the parties. For example, the cost of constructing a dedicated AFR interim storage facility would be prohibitive for a single research reactor but justifiable if it is used by a number of different operators.

5.2.2. Reprocessing of spent fuel

The existing research reactor spent fuel reprocessing options use the PUREX process in which the spent fuel elements are dissolved in nitric acid and the uranium and plutonium they contain are separated and stored as oxides,

and the HLW comprising the fission products and remaining actinides are immobilized in a glass matrix. The process also creates LILW waste streams from process materials, reagents and equipment which can be encapsulated in cement or bitumen and disposed of with other LILW wastes.

The major steps in the PUREX process are:

(a) Head end process — the fuel is dissolved in boiling nitric acid. In some reprocessing schemes, the nitric acid is made more aggressive by addition of hydrofluoric acid, mercury or silver in order to more rapidly dissolve the fuel. Any undissolved residues (including pieces of fuel cladding and fines) are monitored for residual fissile material content and treated as solid waste.

(b) Solvent extraction — uranium and plutonium are separated from the fission products and other actinides using aqueous and organic solvents. The aqueous solvent is nitric acid and the organic phase is tributyl phosphate (TBP) in an organic carrier such as kerosene.

(c) Separation and purification of U and Pu using a second solvent extraction step.

(d) Conversion of uranium and plutonium to oxide forms for storage or re-use.

(e) Concentration and packaging of the liquid and solid high level waste radioactive wastes, high level waste (HLW) and low and intermediate level wastes (LILW) from the initial separation and subsequent processing.

To reduce the ²³⁵U enrichment of less than 2%, the research reactor fuel is reprocessed together with spent fuel from power reactors. The resulting ‘reprocessed uranium’ can then be used in the production of new fuel if appropriate measures are taken to manage the minor uranium isotope content in the reprocessed uranium. The uranium contains the isotopes ²³⁶U, which acts as a thermal neutron absorber, as well as ²³²U and elevated levels of

234U, which present gamma and alpha radiation challenges for the fuel fabrication facilities respectively.

The plutonium can be recycled into fresh mixed oxide (MOX) fuel or can be stored pending a repackaging for final disposal. In this latter case, the small amount of separated plutonium is incorporated within containers of vitrified high level waste.

If reprocessing is selected as the spent fuel management option, the future use of the reprocessed uranium and plutonium has to be defined. The research reactor operator could transfer the ownership title of the uranium and plutonium to a third party for recycling. For example, in a country with a nuclear power programme, the research reactor operator could have an agreement with an electricity utility to transfer the title of the recovered fissile material and agree to a common management of the reprocessing wastes. The financial terms for so doing would depend on national policy, as well as the prevailing price levels within the nuclear fuel markets. At the time of writing this report, the nuclear fuel markets would view the reprocessed uranium as an asset, and the plutonium as a liability.

The advantages of reprocessing spent research reactor fuels include:

(1) A significant reduction in HLW volume of between 30 and 50 times when compared with the option of direct disposal of the research reactor spent nuclear fuels [17], with a potential for reduced costs for eventual repository disposal;

(2) Reduction of the long term radiotoxicity by a factor of 10 compared to direct disposal of spent fuel;

(3) Packaging of the primary waste stream, containing 99% of the radioactivity into a durable form suitable for repository disposal;

(4) Reduction in non-proliferation concerns because of the decreased uranium enrichment;

(5) Reprocessing and conditioning can provide an extended period (~15 years) for the development of other solutions for waste storage;

(6) It is possible to have an agreement with the reprocessing company that includes all phases of the delivery of the reprocessing wastes to the country of origin and management of the recovered fissile material.

Authorization and any necessary licences must be obtained from the relevant authorities in both the research reactor country and the reprocessor country. These include the authorization for cask reception at the reprocessing site, spent fuel unloading and storage, reprocessing of the spent fuel, and the administrative authorizations for the subsequent nuclear material and radioactive waste transfer.

At present, only U-Al and uranium oxide fuel types are accepted for reprocessing. Silicide fuels can also be reprocessed but require greater levels of dilution than U-Al fuels. Consequently, the acceptance conditions are

determined in accordance with the reprocessor’s general schedule, and the cost of silicide fuel reprocessing can be prohibitive because of these technical constraints. However, it is possible that reprocessing methods for silicide fuels will be developed with costs comparable to that of U-Al fuels if these fuels remain in general use.

The cost of reprocessing of research reactor fuel is typically in the range of US $10 000/kg to US $15 000/kg of spent research reactor fuels, excluding transport costs.

A number of operators in high income countries with US origin fuel have chosen not to participate in the US take-back programme or for direct disposal, and instead have opted for spent fuel reprocessing.

5.2.3. Conditioning of spent fuel

Several different spent fuel conditioning technologies are in different stages of development [18, 19], which can be divided into two primary categories:

(1) Those which result in a metallic final product:

• Press and dilute or poison. The spent fuel is mechanically compressed and mixed with either depleted uranium or a neutron poison;

• Chop and dilute or poison. The spent fuel is chopped into small pieces and mixed with depleted uranium or a neutron poison;

• Melt and dilute. The spent fuel is melted, diluted with depleted uranium and then further alloyed with aluminium to form a eutectic (Fig. 12). This process has been developed at the Savannah River Technology Center (SRTC) in the USA.

(2) Those which result in a glass or glass ceramic matrix final product:

• Can-in-canister. A critically safe quantity of non-processed spent fuel is placed in a can which is back-filled with HLW glass to form a solid disposal unit with a radiation barrier to deter unauthorized interference;

• Plasma arc treatment. The spent fuel is co-melted with depleted uranium and neutron absorbers in a plasma centrifugal furnace, and then converted into an HLW glass waste form;

• Glass material oxidation and dissolution system. The spent fuel is placed in a glass melt furnace where it is oxidized by lead dioxide and then converted into a LILW glass waste form;

• Dissolve and vitrify. The spent fuel is dissolved and mixed with depleted uranium to reduce the enrichment, and the mixture is then vitrified as a LILW glass waste form.

Depleted Uranium

Spent Fuel

Assembly Aluminium

Induction

Furnace Offgas

Ingot

< 20% ²³⁵U Storage Canister Geologic Repository

FIG. 12. Process schematic of the melt-dilute process [19].

The advantages of spent fuel conditioning include:

(a) Packaging of the spent fuel into a durable form suitable for repository disposal;

(b) Reduction in non-proliferation concerns because of the decreased uranium enrichment;

(c) Plutonium is not separated from the spent fuel, and therefore presents a lower non-proliferation risk and financial liability.